This controlling device is intended to be put into service only upon incorporation into an engine prime mover system that itself has met the requirements of the above Directive and bears the CE mark.
General Setting Guidelines
When setting the relay characteristic/torque angle for the three phase directional overcurrent
element, a positive angle setting was specified. This was due to the fact that we consider as
polarising voltage, the phase voltage value in quadrature of the current under fault
conditions. With directional earth fault protection, the residual current under fault conditions
lies at an angle lagging the polarising residual earth fault voltage.
The following listed angle settings are recommended to fix the right direction of the earth
fault for the various earthed systems.
Resistance earthed systems 180°
Insulated systems 270°
Petersen Coil system 200°
Transmission Systems (solidly earthed) 90°-120°
The setting ranges for directional/non-directional earth fault and wattmetric protection
(Pe/IeCos) can be found in the FT and TD chapters of the technical guide.
2.4.2 An application of 67N in an Insulated Systems
The advantage with an insulated power system is that during a single phase to earth fault
condition, no high earth fault current will flow. Consequently, it is possible to maintain power
flow on the system even with an earth fault condition present. However, this advantage is
offset by the fact that the resultant steady state and transient over-voltages on the healthy
phases can be very high. In general insulated systems will only be used in low or medium
voltage networks where it does not prove too costly to provide the necessary insulation
against such over-voltages.
Higher system voltages would normally be solidly earthed or earthed via a low impedance.
Operational advantages may be obtained by the use of insulated systems.
However, it is still essential that detection of the earth fault condition is achieved. This is not
possible if standard current operated earth fault protection equipment is used. One possibility
for earth fault detection is by applying a residual over-voltage protection device. This
functionality is provided by the MiCOM P125, P126 and P127 relays.
However, fully discriminative earth fault protection on this type of system can only be
achieved by earth fault protection element. This protection element is included in directional
relays MiCOM P125, P126 & P127.
It is essential that a core balance CT is used for high sensitive earth fault detection. This
eliminates the possibility of spill current that may arise from slight mismatches between
residually connected line CTs. It also enables a much lower CT ratio to be applied, thereby
allowing the required protection sensitivity to be more easily achieved.
